1. Field of Invention
This invention relates to a display device, particularly a thin film transistor driven organic electro-luminescent display device (hereafter referred to as TFT-OELD) which is driven by a thin film transistor (hereafter referred to as TFT) and provided with an organic electro-luminescent element (hereafter referred to as OELD) of a high polymer system formed in a liquid phase process.
2. Description of Related Art
TFT-OELD is promising because it is a display device which realizes light-weightness, thinness, smallness, higher accuracy, wider view angle, lower electric consumption, and the like. FIG. 1 shows a conventional TFT-OELD. FIG. 2 shows a cross-sectional view of the conventional TFT-OELD. Here, there is only one pixel 11 depicted, but there are actually many pixels 11 in plural rows and lines. Here, OELD 18 is a high polymer system, formed by a liquid phase process, such as spin coating, blade coating, ink jet, or the like.
In order to implement gradation, in the case of the structure shown in FIG. 1, a gate voltage of a driving TFT 17 is made to change and conductance is changed, so electric current which flows in the OELD 18 needs to be controlled. However, according to this method, particularly in half tone, irregularity of transistor characteristics of the driving TFT 17 appears as brightness irregularity of the OELD 18, and there is a problem such that the screen becomes non-uniform.
Therefore, as shown in FIG. 3, a method is considered which implements gradation by changing a light emitting area of the OELD 18 (Japanese Patent Application 9-233107). FIG. 4 shows a driving method of this method. A scanning electric potential 31 is applied to a scanning line 12, and a signal line 13 is formed of a signal line (lower bit) 131 and a signal line (upper bit) 132. A signal electric potential (lower bit) 321 and signal electric potential (upper bit) 322 are respectively applied as a signal electric potential 32. A driving TFT 17 is formed of a driving TFT (lower bit) 171 and a driving TFT (upper bit) 172, and the OELD 18 is formed of an OELD (lower bit) 181 and an OELD (upper bit) 182. In this example, 2-bit 4 gradation is considered, so an area ratio between OELD (lower bit) 181 and OELD (upper bit) 182 is 1:2.
In this method, the driving TFT 17 takes either a substantially completely on state or a substantially completely off state. In the on state, the resistance of the driving TFT 17 is small enough to be ignored, compared to the resistance of OELD 18, and the electric current amount which flows in the driving TFT 17 and OELD 18 is substantially determined by only the resistance of the OELD 17.
Therefore, irregularity of transistor characteristics of the driving TFT 17 does not appear as brightness irregularity of the OELD 18. Furthermore, in the off state, voltage applied to the OELD 18 becomes less than a threshold voltage, so the OELD 18 does not emit light at all, and, needless to say, irregularity of transistor characteristics of the driving TFT 17 does not appear as brightness irregularity of the OELD 18.
FIG. 5 is a cross-sectional view of TFT-OELD which implements gradation display by changing a light emitting area of the OELD 18 shown in FIGS. 3 and 4. FIG. 5(a) is a cross-sectional view of the OELD (lower bit) 181, and FIG. 5(b) is a cross-sectional view of the OELD (upper bit) 182. The ratio between the light emitting part 25 of the OELD (lower bit) 181 and the light emitting part 25 of the OELD (upper bit) 182 is preferably 1:2.
A light emitting layer 22 is an OELD of a high polymer system and formed in a liquid phase process. A surface of a bank 24 is lyophobic and the light emitting layer 22 does not remain. Therefore, the area of the OELD 18 is determined by patterning. With respect to a side surface of the bank 24, the materials and processing determine whether the side surface of the bank 24 becomes lyophobic or lyophilic.
FIG. 5 shows the case of a lyophilic side surface of the bank 24. As a phenomenon that is characteristic of a liquid phase process, the light emitting layer 22 has a cross-sectional shape which is pulled toward the side surface of the bank 24. In this case, electric current flows into a thinner part of the light emitting layer 22, and this part becomes a light emitting part 25. The cross-sectional shape of the light emitting layer 22 described here is sensitive to liquid amount, liquid material, an initial position of the liquid, and a state, temperature, atmosphere, or the like of a substrate, and which are difficult to control. That is, it is difficult to obtain an absolute value of a desired light emitting area. Because of this, it is difficult to obtain an accurate ratio of 1:2, between the light emitting part 25 of the OELD (lower bit) 181 and the light emitting part 25 of the OELD (upper bit) 182, and ultimately, it is difficult to obtain accurate gradation.
FIG. 6 is a cross-sectional view of OELD (lower bit) 181 (FIG. 6(a)) and a cross-sectional view of OELD (upper bit) 182 (FIG. 6(b)) in the same manner as in FIG. 5. In FIG. 6, the side surface of the bank 24 is lyophobic. As a phenomenon that is characteristic of a liquid phase process, the light emitting layer 22 has a cross-sectional shape which is distant from the side surface of the bank 24. In this case as well, electric current flows into the thinner part of the light emitting layer 22, and this part becomes the light emitting part 25. In this case as well, in the same manner as in the case of FIG. 5, it is difficult to obtain an accurate ratio of 1:2 between the light emitting part 25 of the OELD (lower bit) 181 and the light emitting part 25 of the OELD (upper bit) 182, so it is difficult to obtain accurate gradation.